The old-school procedure for testing humidity used what is called a sling psychrometer. It features a pair of thermometers, and one is fitted with a cloth sock that is wetted with water. The operator swings the pair of thermometers around a given number of times. The air rushing over the wetted sock reduces the temperature based on humidity. This is the "wet bulb" temperature. Comparing wet- and dry-bulb temperatures produces the percentage of humidity.